Endocrine Abstracts

The thyroid hormone (TH) feedback is the principal mechanism contributing to maintain the circulating levels of thyroid hormones within the physiological limits that are required for an adequate action at the tissue level. Therefore, circulating TSH determination is considered the most accurate parameter to diagnose a disorder of thyroid function. Several defects are known to be associated with the disruption of such mechanism, but their intimate understanding is still limited. Over the past few years, we generated various zebrafish models that faithfully reproduce the biochemical signature described in patients with the aim to obtain insights into the specific alterations of TH feedback. CH is typically characterized by low THs and elevated TSH, but the TSH normalization requires abnormally higher L-T4 doses in several cases (e.g., GLIS3 mutations), thus representing a challenge for clinicians. Interestingly, zebrafish knockdown model for glis3 (glis3KD) displays thyroid dysgenesis associated with low THs and high pituitary tshb signal. In comparison with euthyroid and PTU-induced hypothyroid controls, glis3KD larvae require L-T4 doses of 10- and 3.5-fold higher, respectively, to fully suppress the expression of pituitary tshb. Intriguingly, zebrafish models for SECISBP2 (both mutant and morphant fish) are characterized by normal thyroid development, high T4 and low T3 levels and mild increment of tshb that require a 4-fold higher dose of L-T4 than euthyroid controls for suppression, as likely consequence of D2 activity. Even more complex scenarios emerge with mutations in thyroid hormone receptors (TRs). Zebrafish embryos expressing mutant thrb isoforms, along with those overexpressing the human TRβ2 variants (e.g. E464X), develop an enlarged thyroid gland, elevated T4 and T3 levels, and increased pituitary tshb, which are completely resistant to L-T4 suppression. Remarkably, the overexpression of other TRβ2 variants (e.g., R243Q) exhibits similar biochemical abnormalities, although tshb can be suppressed with L-T4 doses that are 5- and 3-fold higher than those required for euthyroid and PTU-induced hypothyroid controls, respectively. The specificity of TRβ2-dependent regulation of TSH expression is further validated by the absence of defects in the HPT-axis functions in all zebrafish models for RTHα, even when mutant THRA transcripts are injected at the stage of 1-2 cell embryos. However, when a mutated chimeric construct, in which the N-terminus domain of TRα1 is replaced with that of TRβ2, is overexpressed in zebrafish, the levels of tshb and THs appear comparable to those observed in RTHβ. In conclusion, we generated several zebrafish models faithfully reproducing the thyroid function test signatures and refractoriness to TH feedback exhibited by patients with these rare thyroid disorders. The mechanisms contributing such refractoriness are indeed variable and include alterations of TH metabolism or in the TR structures.